Seattle.gov Home Page
Seattle.gov This Department

North Henderson Basins

The North Henderson CSO Reduction Project (view overview map of North Henderson CSO improvements) will reduce the amount of untreated sewage and stormwater that overflows into Lake Washington at two outfalls located offshore of:

Based on flow data, SPU determined that combined sewer overflows (CSO) occur 17 times per year, totaling approximately 2 million gallons. The North Henderson Project will reduce this frequency to no more than one overflow per year per outfall, on average, to meet regulatory requirements. We will do this by adding underground storage at two sites within Seward Park and in the 57th Ave. S. right-of-way near Martha Washington Park. The proposed underground storage facilities would store excess combined sewage and stormwater during major rain events. SPU plans to complete the two new storage facilities in 2018.

Seward Park (Basin 44)

Basin 44 drains toward Seward Park and then south to King County’s Pump Station #10, located at the eastern end of 57th Ave. S. The proposed underground storage facility there will consist of a 2.65 million gallon tank, plus facility vaults to house odor control, mechanical and electrical equipment. The project includes additional piping, outfall replacement, with shoreline work and landscape improvements as determined working with Seattle Department of Parks and Recreation, which owns and manages Seward Park. SPU is recommending to City Council that the underground storage facility be located under the existing tennis courts, which would be rebuilt once the facility is constructed.

57th Ave. S (Basin 45)

Basin 45 also drains to Pump Station 10, which then pumps the combined flows south to King County’s Henderson Pump Station near the corner of Seward Park Ave. S. and S. Henderson St. in Rainier Beach. The Henderson Pump Station ultimately conveys the flows to a King County wastewater treatment plant. The 57th Ave. S. right-of-way is managed by the Seattle Department of Transportation. The proposed underground facility for Basin 45 consists of a 16,000 gallon pipe and piping improvements.

Project Updates

Upcoming meetings will be announced on this page and through our mailing list. Sign up for project updates.

April 18, 2014 Update

The Seattle City Council Committee’s public hearing on Tuesday, May 6th, at 9:30 a.m. to consider both the Partial Transfer of Jurisdiction between City Departments for the preferred project site and the Land Use Decision associated with the Master Use Permit application (DPD #3015640) from SPU to locate an underground storage tank within Seward Park. View the DPD hearing notice.

April 3, 2014 Update

The Seattle City Council's Parks, Seattle Center, Libraries and Gender Pay Equity Committee is scheduled to hold a public hearing on Tuesday, May 6th, at 9:30 a.m. to consider both the Partial Transfer of Jurisdiction between City Departments for the preferred project site and the Land Use Decision associated with the Master Use Permit application (DPD #3015640) from SPU to locate an underground storage tank within Seward Park. View the committee agenda.

March 20, 2014 Update

Surface Restoration Concept Selected

SPU continues to work with the Parks Department on details for the surface restoration of the preferred site under the tennis courts in Seward Park. Three surface restoration concepts were developed for review and are briefly described below. View a graphic that shows each concept.

The “Boardwalk” concept provides wood-textured concrete surfaces on paved areas that improve aesthetics and provides a boardwalk-like appearance to the site. This concept balances park user experience with long term access, durability for operations, and maintenance through the use of concrete surfaces.

The “Shoreline” concept is similar to the Boardwalk concept, except that it proposes a concrete treatment that has a wave pattern with seeded aggregate and/or colored concrete. This visually relates to the lake and shoreline, provides durable access for operations, and has low maintenance requirements.

The “Pervious” concept explores incorporation of more grass and plantings and reduces concrete (impervious) surfaces as much as possible. Concerns around this concept include difficulty of establishing landscaping in shallow soil areas over the buried tank, health of turf in areas of frequent pedestrian and vehicular use, and increased levels of Parks resources required for maintenance overall.

The “Shoreline” concept has been selected by Parks and SPU as the preferred option for this project and the departments are working now toward a final design. The goal remains to create an accessible, aesthetic, and durable design that can be reasonably maintained.

January 28, 2014 Update

The stakes are in place for the corners of tennis courts, tank, and limits of construction in Seward Park. The tree flagging indicates trees currently planned to be removed for construction. Parks surveyors planned to remove the tree flagging by Friday, February 7, 2014. The corners were marked using duct tape, paint, and stakes depending on the location. View boundary locations (pdf) marked in the field.

Recent Documents

November 2013 - Frequently Asked Questions (pdf)

October 2013 - Revised Final Environmental Impact Statement Addendum (pdf)

September 2013 - Revised Final Environmental Impact Statement (pdf)

Site Selection and Public Process

Through of series of meetings, workshops, email updates, one-on-one conversations, briefings and direct mailings starting in 2010, we have engaged the community and kept stakeholders informed about the North Henderson Project throughout the site selection process. See Project Background [link to new page] for more details.

The process resulted in these outcomes:

  • 2.65 million gallon underground storage facility beneath the tennis courts in Seward Park
  • 16,000 gallon storage pipe in 57th Ave. S. near SPU’s Pump Station 10, next to Martha Washington Park

In a series of four public workshops, SPU sought input on its evaluation of types of control options and then on four conceptual solutions. Key workshops and decision points were:

  • sewer separation/green infrastructure
  • tunnel storage
  • pump and storage in Martha Washington Park
  • separate storage for each basin, considering street right-of-way, private property and park property

Separate storage facilities in Seward Park and in or near Martha Washington Park were selected as:

  •  the least disruptive to the communities during construction
  • the best fit to meet technical requirements
  • having the least environmental impacts, and
  • the most likely to meet the regulatory schedule mandated by the Washington Dept. of Ecology and the U.S. EPA.

Storage Facility Beneath the Tennis Courts In Seward Park (Basin 44)

In the June 7, 2011 Draft EIS Scoping meeting and the October 8, 2012 public hearing, SPU sought input on two site-specific alternatives for storage in Seward Park (Basin 44): beneath the tennis courts or beneath Parking Lot #2. SPU analyzed the environmental impacts of each alternative through the EIS process and is recommending building the underground storage facility at the tennis court location because:

  •  Seward Park is a destination park, and storage under the tennis courts would have less short-term (construction) and long-term impacts on recreation in Seward Park than storage under Parking Lot #2, and
  •  Other environmental impacts were not significant as defined by State Environmental Policy Act (SEPA), i.e., there was not a reasonable likelihood of more than a moderate adverse impact on environmental quality.

The City Council must still approve the selected alternative in Seward Park. This decision is expected in 2014.

Storage Pipe in 57th Ave. S. (Basin 45)

In the June 9, 2011 workshop, SPU sought input on alternatives for locating Basin 45 storage in the 57th Ave. S. right-of-way, nearby private property and in Martha Washington Park. SPU followed the community’s suggestion to contact private property owners, located a willing seller and purchased the home at 5560 S. Holly St. for the proposed 200,000 gallon storage facility. SPU received additional input from the community on the conceptual storage design in the February 26, 2012 open house.

Subsequent analysis demonstrated that CSO control could be achieved by simplifying the Basin 45 project from a 200,000 gallon storage tank to a smaller, 16,000 gallon storage pipe, some repiping in the 57th Ave. S. right-of-way and retaining more of Basin 44 flows in Basin 44. This solution results in a slight increase (from 2.4 to 2.65 million gallons) in storage under the Seward Park tennis courts without increasing the outside dimensions of the storage tank. This change eliminates the need for the storage facility at 5560 S. Holly St. and will be significantly less disruptive to the community. SPU completed a Final EIS Addendum process in October 2013 that documents the environmental analysis associated with this change.

Project Materials

Past Meetings and Materials

References and Final Reports

Project Schedule

  • July 15, 2013
    30% Design Completion (Complete)
  • Issued September 5, 2013
    Revised Final Environmental Impact Statement (Complete)
  • Issued September 30, 2013
    Revised Final Environmental Impact Statement Addendum (Complete)
  • December 2013
    60% Design Completion (In Progress)
  • June 2014
    90% Design Completion
  • November 2014
    Final Design Completion
  • 1st Quarter 2015
    Begin Construction (Basin 44)*
  • 1st Quarter 2017
    End Construction (Basin 44)*
    * Construction of Basin 45 improvements will require approximately 6 months and will occur during the construction of Basin 44.
  • 1st Quarter 2018
    Commissioning and Testing of Facilities Completion

Contact Us

Your continued input is important to us. To help the project team best understand your concerns and interests, please specify the project site with your comments: Seward Park (Basin 44) or 57th Ave. S. right-of-way (Basin 45).

Project Contact
Alan Lord
(206) 233-1565
Alan.Lord@seattle.gov

Sign up for project updates at seattle.gov/lists/cso-northhenderson.htm.

For interpretation services, please call (206) 233-1565.
Para servicios de interpretación, por favor llame al (206) 233-1565.
Về dịch vụ phiên dịch xin gọi (206) 233-1565.
Hadii aad u baahantahay turjubaan, fadlan inaga soo wac telefoonkan (206) 233-1565.
如需要口譯服務,請撥電話號碼 (206) 233-1565.